Light-emitting element, production method thereof, and light-emitting apparatus

ABSTRACT

The present invention provides an light-emitting element in which an organic compound layer containing a carbonate, for example Cs 2 CO 3  and Li 2 CO 3 , as a dopant is in substantially electrical contact with a cathode by providing an organic compound layer having a dopant easy in handling so as to bring the organic compound layer into contact with the cathode. The light-emitting element includes a pair of electrodes sandwiching the organic compound layer, which is a co-evaporation layer of an organic compound and the carbonate.

TECHNICAL FIELD

The present invention relates to a light-emitting element which isprovided with at least one layer an organic compound between an anodeand a cathode, and a production method thereof, and a light-emittingapparatus having the light-emitting element.

BACKGROUND ART

A light-emitting element is a so-called organic electroluminescentelement in which, by an electric current flowing between the cathode andanode, an organic compound between both electrodes is made to emitlight.

A general sectional structure of a light-emitting element is shown inFIG. 1. In the FIG. 1, 1 denotes a transparent substrate, 2 atransparent electrode (anode), 3 a hole transporting layer, 4 alight-emitting layer, 5 an electron transporting layer, 6 an electroninjecting layer, and 7 a cathode.

In this light-emitting element, an exciton is generated by therecombination of the electron, injected to the light-emitting layer 4from the cathode 7 through the electron injecting and transportinglayers 5 and 6, and the hole injected to the light-emitting layer 4 fromthe transparent electrode 2 through the hole transporting layer 3. Thelight-emitting is an element which takes advantage of the light emittedwhen the exciton returns to the ground state.

For the cathode 7 of such a light-emitting element, there is used amaterial which has a relatively small work function and satisfactoryelectron injection characteristics, for example, an elemental metal suchas magnesium (Mg) or a metal alloy such as Ag—Mg and Al—Li alloys.

In addition, patent document 1 discloses a configuration in which anorganic layer containing a metal functioning as a donor (electrondonating) dopant is provided in contact with the cathode. As the metalused as the donor (electron donating) dopant, patent document 1discloses alkali metals, alkaline earth metals, and transition metalsinclusive of rare earths.

In addition, patent document 2 discloses a configuration in which anorganic layer having a metal oxide or a metal salt as a dopant isprovided in contact with the cathode.

Patent document 1: Japanese Patent Application Laid-Open No. 10-270171(page 2, lines 9 to 13, and FIG. 1).

Patent document 2: Japanese Patent Application Laid-Open No. 10-270172(page 2, lines 2 to 7, and FIG. 1).

As these dopants, metals each having a work function as small aspossible, and the oxides and metal salts containing such metals arepreferable. Such a metal is generally high in reactivity, and hencehandling such a metal is very tough. In addition, when a metal oxide ora metal salt containing such a metal is used as a dopant, the dopantbecomes higher in stability, but still some of the metal oxides andmetal salts are unstable so that handling thereof in the usual airenvironment is not easy. On the other hand, some others of the metaloxides and metals salts can be handled in the air, but they are stillunstable so that it is difficult to introduce them each as a dopant intothe organic layer provided in contact with the cathode, as the case maybe. Accordingly, the uneasy handling of them as the dopants can be acause for the low process yield and the cost rise in the production ofthe light-emitting element.

DISCLOSURE OF THE INVENTION

The present invention provides a light-emitting element having a dopanteasy in handling and the production method thereof.

Specifically, the present invention provides a light-emitting elementcomprising: a pair of electrodes consisting of an anode and a cathode,and an organic compound layer provided between the pair of electrodes,wherein the organic compound layer in substantially electrical contactwith the cathode is composed of at least an organic compound and acarbonate, and a molar ratio of the organic compound to the carbonate inthe organic compound layer is in a range from 1:0.01 to 1:100.

In addition, the present invention provides a production method of alight-emitting element comprising a pair of electrodes consisting of ananode and a cathode, and an organic compound layer provided between thepair of electrodes, which method comprises: an organic compound formingstep of providing at least an organic compound and a carbonate forconstituting the organic compound layer, on the side of the cathode toform the organic compound layer, and a step of providing the cathode tocome into substantially electrical contact with the organic compoundlayer, wherein the organic compound layer forming step is the step offorming the organic compound layer in a molar ratio of the organiccompound to the carbonate ranging from 1:0.01 to 1:100.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of the stacked layerconfiguration in a general light-emitting element;

FIG. 2 is a schematic diagram showing an example of the stacked layerconfiguration of a light-emitting element in the present embodiment;

FIG. 3 is a graph showing the voltage-luminance characteristics of thelight-emitting elements in the examples and comparative examples of thepresent invention;

FIG. 4 is a graph showing the voltage-luminance characteristics of thelight-emitting elements in the examples and comparative examples of thepresent invention;

FIG. 5 is a schematic diagram showing the third example;

FIG. 6 is a schematic diagram showing the stacked layer configuration inthe light-emitting element of the fourth example; and

FIG. 7 is a graph showing the voltage-luminance characteristics in thefourth example and the eighth comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

The light-emitting element of the first embodiment of the presentinvention is a light-emitting element which has at least a pair ofelectrodes (an anode and a cathode) and a light-emitting layerinterposed between the pair of electrodes, wherein a dopant in anorganic compound layer in contact with the cathode is a carbonate. Inother words, the attention is paid to an anion in the salt. A carbonateis easy in handling. Among the carbonates, it is preferable that thecarbonate is a carbonate of an alkali metal or a carbonate of analkaline earth metal in view of the easiness in handling. Needless tosay, the respective carbonates (of the alkali metals and alkaline earthmetals) may be simultaneously contained in the organic compound layer,or there may be mixed other additives facilitating the electroninjection or transportation in addition to the organic compound and thecarbonate.

As a reason why the carbonate is preferable, there may be said theimprovement in the durability of an element to be obtained. Thedurability means the element life duration. This is because that thecarbonate is relatively large in molecular weight so that it is expectedthe carbonate is difficult to migrate in the organic compound layer whenthe element is driven to operate. Furthermore, the satisfactory affinityof the carbonate with the organic compound constituting the organiccompound layer may be said as a reason why the carbonate is preferablyused.

Under the favor of the organic compound layer, electrons are efficientlysupplied from the cathode to the light-emitting layer. Consequently, inthe present embodiment, when a metal material to be used in the cathodeis chosen, there is no limitation caused by consideration of the workfunction of the material; in other words, even when there are used suchelectrode materials having relatively high work functions as ITO, gold,silver, and the alloys thereof, the satisfactory electron injection intothe light-emitting element becomes possible.

In the present embodiment, the carbonate which can particularlypreferably be used is at least any one of cesium carbonate (Cs₂CO₃) andlithium carbonate (Li₂O₃). Among others, the light-emitting element, inwhich ITO is used as the cathode and either cesium carbonate or lithiumcarbonate is used as the dopant, has a satisfactory opticaltransmittance, so that light can be brought out from the cathode, and itis suitable as a so-called top emission type light-emitting element.Needless to say, a light-emitting element of the present invention maybe a light-emitting element having a mode in which light is brought outfrom the anode. Incidentally, the organic compounds may be those wellknown in the art, and there can be cited, for example, Alq3 as shown inExample 1. The optical transmittance of the organic compound layer incontact with the cathode is 80% or above in the wavelength range from450 nm to 700 nm (that is, 450 nm or above and 700 nm or below; the sameis applied hereinafter), and furthermore 95% or above.

In addition, the film thickness of the organic compound layer containinga carbonate falls in the range from 0.1 to 10000 nm, preferably in therange from 1 to 500 nm. Incidentally, the film thickness of thelight-emitting layer can have an arbitrary value. In order to obtain ahigh light-emitting efficiency, it is preferable that there is asufficient recombination region along the film thickness direction inthe light-emitting layer; the film thickness of the light-emitting layeris generally of the order of from 15 to 20 nm. A light-emitting elementaccording to the present embodiment may have a light-emitting layerhaving a film thickness of approximately such an order of magnitude.

In a light-emitting element of the present embodiment, the main organiccompound constituting an organic compound layer is a small molecular(monomer) compound.

The small molecular (monomer) compound is defined in the presentinvention as an organic compound which has a molecular weight of 2000 orbelow, more preferably 1000 or below.

The specific types of such organic compounds will be described below.

The quantitative ratio of the organic compound and the carbonate isdescribed below.

The quantitative ratio of the organic compound to the carbonate fallswithin the range from 1:0.01 to 1:100 in molar ratio. Preferably theratio falls within the range from 1:0.1 to 1:10. The molar ratio asreferred to here means the ratio of the number of moles of the organiccompound introduced to the organic compound layer to the number of molesof the carbonate. In particular, in the case of using the carbonatecontaining an alkaline metal as cation in the molar ratio of 1:0.5,electrons can be supplied efficiently from the cathode to thelight-emitting layer.

Incidentally, as for the above ratio of 1:0.5, a margin of deviation ofthe order of approximately 20% are acceptable; that is, the particularlypreferable molar ratio falls within the range from 1:0.4 to 1:0.6. Thereason why the effect becomes best for the molar ratio of 1:0.5 cannotbe stated clearly at present; it is conceivable that such a ratio is aquantitative ratio relationship suitable for the interaction between theorganic compound and the carbonate.

In the case of using the carbonate such as cesium carbonate in theorganic compound layer, even when alminum or a transparent conductiveoxide such as ITO is used as the cathode, good electron injection fromthe cathode to the organic compound layer is realized to obtain aremarkably good result of the light-emitting efficiency of thelight-emitting element. Among them, the element using ITO is preferablyused as the top emission type light-emitting element in which light istaken out from one electrode of the pair of electrodes opposing to thesubstrate side toward the outside of the element.

That is, the present invention can provide a light-emitting elementhaving the features of using the organic compound layer containing thecarbonate and using the transparent conductive oxide such as ITO and IZOas the cathode. The present invention can be also provide a top emissiontype light-emitting element having the above-described features.

On the other hand, the element having a single layer of a conventionallyused material such as lithium fluoride in place of the organic compoundlayer, and the element having the organic compound layer containing thematerial such as lithium fluoride exhibit inferior electron injection inany case of using aluminum and a transparent conductive oxide as thecathode. Among them, in the case of using the transparent conductiveoxide as the cathode, the electron injection scarely occurs, andtherefore the electron injection of the element is remarkably bad.

In addition, another layer may be provided between the cathode and theorganic compound layer. The another layer may be an organic layer, aninorganic layer, or a mixed layer of organic and inorganic compounds. Tobe more specific, it may be a LiF layer. Incidentally, by providing suchanother layer, the electron injection is further improved. Even with theanother layer being provided, it can be said that the cathode and theorganic compound layer is in substantially electrical contact with eachother.

When an organic compound layer composed of an organic compound and acarbonate is formed, it is preferable to co-evaporate both of them. Inparticular, it is preferable that the organic compound layer is formedwhile the carbonate being in a heated state. The formation of theorganic compound layer while the carbonate being in a heated state makesthe current density of the light-emitting element to reach a practicallypreferable level. When the carbonate is heated to be used, the carbonatesuch as cesium carbonate and lithium carbonate can be heated atapproximately from 150° C. or above to 700° C. or below. The temperaturerange is a relatively lower temperature range. Additionally, in such atemperature range, the carbonate can be handled simultaneously with theorganic compound while both being heated. The temperature region (range)may fall in any of the temperature regions of the melting point,decomposition point, and decomposition starting point of the carbonate.For example, it is preferable that the decomposition starting point ofcesium carbonate is about 610° C.; and it is also preferable that thedecomposition starting point of lithium carbonate is 615° C.

The reason why it is preferable to heat the carbonate cannot be statedpositively at present, it is conceivable that the heating is preferablefor the purpose of attaining the above described interaction.

Alternatively, when the carbonate is heated, the carbonate may reducethe organic compound simultaneously. This may make possible it todecrease the barrier for the electron injection from the cathode,thereby lowering the driving voltage of the element, even when suchstable metals as gold (Au) and silver (Ag), and a transparent ITOelectrode are used.

In addition, the carbonate (for example, Cs₂CO₃) is more preferable ascompared to the elemental metal (for example, the elemental metal ofcesium). It is conceivable that this may be because the carbonate hasthe larger molecular weight (that is, heavier weight) as compared to theelemental metal of cesium, whereby the carbonate makes the migration ofthe cesium element difficult.

The film formation of the organic compound layer may be carried out byany film formation method. For example, an evaporation method and asputtering method can be used. The carbonates can be heated in thesemethods, and hence are preferable methods.

The present embodiment is described below in more detail and morespecifically.

As a material doped in the organic layer in contact with the cathode,the present inventors discovered a specific material which is easilyavailable, which is handled without necessity of a special workingenvironment for eliminating the contact with the air and the moisture,and furthermore which can be applied to the film formation with thegeneral methods such as the resistive heating and the like. That is, thediscovered material the carbonate.

In addition, the light-emitting element of the present embodiment can beapplied to a display device such as a light-emitting element arrayhaving a plurality of light-emitting elements and a display(irrespective as to whether monocolor or full color), and a light sourcefor exposure of a photosensitive member in an electrophotography system(for example, a laser beam printer and a copying machine).

In addition, in the light-emitting element of the embodiment of thepresent invention, a material used for the cathode, can include aluminum(Al), silver (Ag), gold (Au), and indium tin oxide (ITO), or an alloycontaining at least one of these metals, and the like. In addition tothese, there can be used magnesium (Mg), platinum (Pt), palladium (Pd),selenium (Se), iridium (Ir), tin oxide, and copper iodide, or a mixedmetal (for example, an alloy) containing at least one of these metals.

The light-emitting element of the embodiment of the present invention isnot limited as to the order of the film formation for the cathode andthe organic compound layer in contact with the cathode, and the order ofthe above-described film formation can be chosen without restraint.

Second Embodiment

FIG. 2 is a schematic diagram showing the second embodiment of thepresent invention. The present embodiment is an embodiment in which thedescription of a portion of the light-emitting element of the firstembodiment is extended to a description covering a layer configurationincluding the anode. In FIG. 2, the light-emitting element of thepresent invention is constituted by stacking, on a substrate 10, anelectrode 11 for forming an anode, a hole transporting layer 12 having ahole transporting property, a light-emitting layer 13, an organiccompound layer 14 composed of an organic compound and a carbonate, andan electrode 15 for forming a cathode.

In addition to the above, the configuration of the above-describedorganic compound layer can include the following configurations:electrode (anode)/light-emitting layer/organic compound layer/electrode(cathode); electrode(anode)/hole transporting layer/light-emittinglayer/electron transporting layer/organic compoundlayer/electrode(cathode); electrode(anode)/hole injectinglayer/light-emitting layer/organic compound layer/electrode(cathode);electrode(anode)/hole injecting layer/hole transportinglayer/light-emitting layer/organic compound layer/electrode(cathode);and electrode(anode)/hole injecting layer/hole transportinglayer/light-emitting layer/electron transporting layer/organic compoundlayer/electrode(cathode). The light-emitting element according to thepresent invention can have any element configuration as far as theorganic compound layer 14 is provided on the interface with the cathode15. Furthermore, specifically, it is desirable that the layerconfiguration has an order of the cathode, organic compound layer, andelectron transporting layer (needless to say, as for the productionsequential order, sometimes the layers are formed in the order of theelectron transporting layer, organic compound layer, and cathode). Inparticular, in the case of this layer configuration, there can besuitably used, as the electron transporting layer, at least any of themetal complex compounds such as Alq3 and PBO, and the heterocycliccompounds and fused heterocyclic compounds such as oxazole, triazole,quinoxaline, triazine, and silole. By further providing such an electrontransporting layer, the efficiency for the electron transportation fromthe cathode to the light-emitting layer is further improved. In thiscase, the material for the electron transporting layer and the organiccompound mainly constituting the organic compound layer may be differentcompounds, but it is preferable that the material and the main organiccompound is the same compound.

As for the compounds which can be used as the hole transporting layer 12and the hole injecting layer, there is no particular limitation; forexample, triphenyldiamine derivatives, oxadiazole derivatives, porphyrylderivatices, stilbene derivatives, and the like can be used, but thereis no limitation to these compounds.

As for the compound which can be used as the material for thelight-emitting layer 13, it can be adopted from triarylaminederivatives, stilbene derivatives, polyarylene, fused polycyclicaromatic compounds, heterocyclic aromatic compounds, fused heterocyclicaromatic compounds, metal complex compounds, and the like, and thehomo-oligomers thereof, the composite oligomers thereof, or the like. Inaddition, one or more kinds of these light-emitting materials can beused and doped in the hole injecting layer, hole transporting layer, orelectron transporting layer. These materials and configurations are notrestricted to these.

As the electrode 11 for forming the anode, an electrode having a largework function is preferable; for example, there can be used indium tinoxide (ITO), tin oxide, gold (Au), platinum (Pt), chromium (Cr),palladium (Pd), selenium (Se), iridium (Ir), copper iodide, and thelike, and alloys and the like.

The above-described hole transporting layer 12, hole injecting layer,light-emitting layer 13, and electron transporting layer may be producedby using any types of film forming methods; for example, there can beused the evaporation method, sputtering method, CVD method, molecularbeam evaporation method (MBE method), dipping method, spin coatingmethod, casting method, bar coat method, roll coat method, ink jetmethod, and the like.

In addition, in the light-emitting element described in the presentembodiment of the present invention, it is possible to take aconfiguration in which the element is protected from the oxygen andmoisture by providing a protection layer made of an organic material oran inorganic material, without introducing any adverse effects on thecharacteristics of the present invention. In addition, it is alsopossible to improve environmental resistance of the element by enclosingthe element with an inert gas.

Third Embodiment

The light-emitting element according to the third embodiment of thepresent invention has a configuration in which an additional layer isprovided between the organic compound layer and the light-emittinglayer. Except this, the present embodiment is the same as the secondembodiment. As for the present embodiment, among the layerconfigurations described in the second embodiment, the layerconfigurations such as the configuration of electrode (anode)/holetransporting layer/light-emitting layer/electron transportinglayer/organic compound layer/electrode (cathode) and the configurationof electrode (anode)/hole injecting layer/hole transportinglayer/light-emitting layer/electron transporting layer/organic compoundlayer/electrode (cathode) are included in the category of the presentembodiment. In addition to these configurations, there can be provided ablock layer between the light-emitting layer and the organic compoundlayer, to be more specific, a layer functioning as a hole blockinglayer. Incidentally, the hole blocking layer may have the ability of theelectron transporting ability or the electron injecting ability. Thefollowing descriptions in an itemized form are prepared for the blocklayer, and any of the following items from A to F is acceptable.

-   A. As a material constituting the block layer, a metal complex    compound can be used.-   B. As a material constituting the block layer, a heterocyclic    compound can be used.-   C. As a material constituting the block layer, a fused heterocyclic    compound can be used.-   D. As a material constituting the block layer, an aluminum chelate    complex (Alq3) can be used.-   E. As a material constituting the block layer, bathophenathroline    can be used.-   F. As a material constituting the block layer, bathocuproin can be    used.

EXAMPLES

Description will be made below on the preferred examples of the presentinvention, by referring to the drawings, with cesium carbonate as anillustrative example of the carbonate, but lithium carbonate can also beapplied, that is, the present invention is not limited to the presentexamples.

Example 1

Example 1 is illustrated in FIG. 2. In FIG. 2, numeral 10 denotes atransparent substrate, 11 an ITO layer as an anode electrode for use inhole injection, 12 a hole transporting layer, 13 a light-emitting layer,14 an organic compound layer, and 15 a cathode electrode.

A film of indium tin oxide (ITO) of 120 nm in thickness was formed bythe sputter method on the transparent substrate 10, and the transparentanode electrode 11 was obtained. Subsequently, the substrate wassubjected to ultrasonic cleaning successively with acetone and isopropylalcohol (IPA), and then cleaned in boiling IPA and dried. Furthermore,the substrate was subjected to UV/ozone cleaning.

Then, using a vacuum evaporation apparatus (manufactured by Shinku-Kiko,Inc.), on the cleaned substrate, α-NPD having hole transporting propertyrepresented by the following chemical formula. 1:

was evaporated by the vacuum evaporation method to form a film of 35 nmin thickness as the hole transporting layer 12. The film was formedunder the conditions that the degree of the vacuum when evaporating was1.0×10⁻⁶ Torr, and the film formation rate was from 0.2 to 0.3 nm/sec.Then, on the hole transporting layer 12, an aluminum chelate complex(hereinafter referred to as “Alq3”) represented by the followingchemical formula 2:

was evaporated by the vacuum evaporation method to form a film of 15 nmin thickness as the light-emitting layer 13 under the same conditions asthose for the film formation of the hole transporting layer 12. Then, onthe light-emitting layer 13, Alq3 and cesium carbonate (Cs₂CO₃) wereevaporated to form a film of 35 nm in thickness as the organic compoundlayer 14 in such a way that the evaporation rates of the Alq3 and cesiumcarbonate were respectively so adjusted that the Alq3 and cesiumcarbonate were mixed in the film thickness ratio of 9:1. Finally,aluminum (Al) was evaporated to form the cathode electrode 15 having athickness of 150 nm on the organic compound layer 14 under the conditionof the evaporation rate of 1 nm/sec. The molar ratio is a ratiocalculated from the molecular weight and specific gravity of thematerials for forming the organic compound layer., and the ratio of filmthicknesses. The molar ratio of Alq3 to cesium in the organic compoundlayer was 1:0.57.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 10, the anode electrode 11, hole transportinglayer 12, light-emitting layer 13, organic compound layer 14, andcathode electrode 15. Successively, while a direct current voltage wasapplied between the ITO as the anode electrode 11 and the aluminum asthe cathode electrode 15 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 41200 cd/m²and a current density of 3200 mA/cm² at an applied voltage of 15 V. Inaddition, the element exhibited a maximum efficiency of 0.91 lm/W at anapplied voltage of 5 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 3 and FIG. 4.

Comparative Example 1

The present comparative example is different from Example 1 in the pointthat carbonate is not used.

Under the conditions similar to those in Example 1, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 12 on the ITO as the anode electrode 11, and thereon an Alq3 filmof 50 nm in thickness was formed as the light-emitting layer 13.Finally, aluminum (Al) was evaporated in a thickness of 150 nm as thecathode electrode 15.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 10, the anode electrode 11, hole transportinglayer 12, light-emitting layer 13, organic compound layer 14, andcathode electrode 15. Successively, while a direct current voltage wasapplied between the ITO as the anode electrode 11 and the aluminum asthe cathode electrode 15 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 1926 cd/m²and a current density of 350 mA/cm² at an applied voltage of 20 V. Inaddition, the element exhibited a maximum efficiency of 0.17 lm/W at anapplied voltage of 12 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 3 and FIG. 4.

From comparison of the voltage-luminance characteristics of Example 1and Comparative Example 1 shown in FIG. 3, the light-emitting elementshown in Example 1 which used cesium carbonate in the organic compoundlayer is seen to be largely decreased in the driving voltage as comparedwith the element of Comparative Example 1. Accordingly, the organiccompound layer is seen to be effective in lowering the driving voltageof the element.

Comparative Example 2

The present comparative example shows that the film formation ofmetallic cesium, tough in handling, was attempted by the same process asthat in Example. 1, but it could not carried out.

Under the conditions similar to as those in Example 1, at the beginning,an α-NPD film of 35 nm in thickness was formed as the hole transportinglayer 12 on the ITO as the anode electrode 11, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 13. Then,as the organic compound layer 14, Alq3 and cesium (Cs) were attempted tomix with each other in the film thickness ratio of 9:1, but the metalliccesium high in reactivity could not be introduced into a film formingapparatus in the atmospheric environment so that the film could not beformed. Thus, in order to produce an element in which an alkali metalsuch as cesium (Cs) is introduced into the organic compound layer, thereis needed a special work environment in which the alkali metal can behandled and the film thereof can be formed under the condition ofprohibiting the contact with the air and moisture. The construction ofsuch a work environment needs a high cost, and the time taken forproducing an element becomes longer than that in the usual environment,and hence the element production throughput is lowered.

Comparative Example 3

In the present comparative example, a special apparatus was used for thepurpose of using the cesium metal tough in handling.

Under the same conditions as those in Example 1, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 12 on the ITO as the anode electrode 11, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 13. Then,it was attempted that as the organic compound layer 14, Alq3 and cesium(Cs) were mixed to form a mixed film of 35 nm in thickness. In thepresent Comparative Example 3, the introduction of cesium into theorganic compound layer 14 was performed using an alkali metal dispenser(manufactured by SAES Getter, Inc.) which could generate a high purityof alkali metal vapor in vacuum. Alq3 and cesium (Cs) were evaporated toform a film of 35 nm in thickness as the organic compound layer 14 insuch a way that the evaporation rates of the Alq3 and cesium wererespectively so adjusted that the Alq3 and cesium were mixed in the filmthickness ratio of 9:1. Finally, aluminum (Al) was evaporated to formthe cathode electrode 15 in a thickness of 150 nm on the organiccompound layer 14 under the condition of the evaporation rate of 1nm/sec.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 10, the anode electrode 11 hole transporting layer12, light-emitting layer 13, organic compound layer 14, and cathodeelectrode 15. Successively, while a direct current voltage was appliedbetween the ITO as the anode electrode 11 and the aluminum as thecathode electrode 15 in the light-emitting element, the light-emittingcharacteristics of the element were measured. Consequently, the elementexhibited a maximum luminance of 11000 cd/m² and a current density of3085 mA/cm² at an applied voltage of 12 V. In addition, the elementexhibited a maximum efficiency of 0.47 lm/W at an applied voltage of 5V. The voltage-luminance characteristic of the light-emitting element isshown in FIG. 3.

Comparative Example 4

In the present comparative example, a salt other than the carbonates wasused.

Under the conditions similar to those in Example 1, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 12 on the ITO as the anode electrode 11, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 13. Then,Alq3 and lithium fluoride (LiF) were evaporated to form a film of 35 nmin thickness as the organic compound layer 14 in such a way that theevaporation rates of the Alq3 and lithium fluoride (LiF) wererespectively so adjusted that the Alq3 and lithium fluoride were mixedin the film thickness ratio of 9:1. Finally, aluminum (Al) wasevaporated to form the cathode electrode 15 in a thickness of 150 nm onthe organic compound layer 14 under the condition of the evaporationrate of 1 nm/sec.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 10, the anode electrode 11, hole transportinglayer 12, light-emitting layer 13, organic compound layer 14, andcathode electrode 15. Successively, while a direct current voltage wasapplied between the ITO as the anode electrode 11 and the aluminum asthe cathode electrode 15 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 23900 cd/m²and a current density of 2450 mA/cm² at an applied voltage of 18 V. Inaddition, the element exhibited a maximum efficiency of 0.78 lm/W at anapplied voltage of 5 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 3 and FIG. 4.

Comparative Example 5

In the present comparative example, lithium fluoride (LiF) was used inplace of the organic compound layer composed of a carbonate and anorganic compound in Example 1.

Under the conditions similar to those in Example 1, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer on the ITO as the anode electrode, and thereon an Alq3 film of 50nm in thickness was sequentially formed as the light-emitting layer.Then, lithium fluoride (LiF) was evaporated to form a film of 1 nm.Finally, aluminum (Al) was evaporated to form the cathode electrode in athickness of 150 nm under the condition of the evaporation rate of 1nm/sec.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate, the anode electrode, hole transporting layer,light-emitting layer, lithium fluoride (LiF), and cathode electrode.Successively, while a direct current voltage was applied between the ITOas the anode electrode and the aluminum as the cathode electrode in thelight-emitting element, the light-emitting characteristics of theelement were measured. Consequently, the element exhibited a maximumluminance of 26790 cd/m² and a current density of 2188 mA/cm² at anapplied voltage of 18 V. In addition, the element exhibited a maximumefficiency of 0.86 lm/W at an applied voltage of 5 V. Thevoltage-luminance characteristic of the light-emitting element is shownin FIG. 3 and FIG. 4.

Example 2

The present example is an example in which gold is used for the cathodeelectrode contrary to Example 1.

Under the conditions similar to those in Example 1, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 12 on the ITO as the anode electrode 11, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 13. Then,Alq3 and cesium carbonate (Cs₂CO₃) were evaporated to form a film of 35nm in thickness as the organic compound layer 14 in such a way that theevaporation rates of the Alq3 and cesium carbonate were respectively soadjusted that the Alq3 and cesium carbonate were mixed in the filmthickness ratio of 9:1. Finally, gold (Au) was evaporated to form thecathode electrode 16 in a thickness of 150 nm on the organic compoundlayer 14 under the condition of the evaporation rate of 1 nm/sec. Themolar ratio of Alq3 to cesium carbonate in the organic compound layerwas 1:0.57.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 10, the anode electrode 11, hole transportinglayer 12, light-emitting layer 13, organic compound layer 14, andcathode electrode 15. Successively, while a direct current voltage wasapplied between the ITO as the anode electrode 11 and the gold (Au) asthe cathode electrode 15 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 29700 cd/m²and a current density of 3000 mA/cm² at an applied voltage of 15 V. Inaddition, the element exhibited a maximum efficiency of 0.79 lm/W at anapplied voltage of 6 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 4.

Comparative Example 6

In the present comparative example, lithium fluoride was used instead ofa carbonate used in Example 2.

Under the conditions similar to those in Example 2, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 12 on the ITO as the anode electrode 11, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 13. Then,Alq3 and lithium fluoride (LiF) were evaporated to form a film of 35 nmin thickness as the organic compound layer 14 in such a way that theevaporation rates of the Alq3 and lithium fluoride were respectively soadjusted that the Alq3 and lithium fluoride were mixed in the filmthickness ratio of 9:1. Finally, gold (Au) was evaporated to form thecathode electrode 15 in a thickness of 150 nm on the organic compoundlayer 14 under the condition of the evaporation rate of 1 nm/sec.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 10, the anode electrode 11, hole transportinglayer 12, light-emitting layer 13, organic compound layer 14, andcathode electrode 15. Successively, while a direct current voltage wasapplied between the ITO as the anode electrode 11 and the gold (Au) asthe cathode electrode 15 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 93 cd/m² anda current density of 26 MA/cm² at an applied voltage of 25 V. Inaddition, the element exhibited a maximum efficiency of 0.047 lm/W at anapplied voltage of 24 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 4.

When lithium fluoride is used in the organic compound layer, theelectron injecting characteristic becomes good by using the cathode madeof aluminum, but the electron injecting characteristic is degraded byusing the cathode made of gold. On the contrary, when a carbonate isused, a satisfactory electron injecting characteristic is obtained byusing either the cathode made of aluminum or the cathode made of gold,in other words, the degree of freedom in choosing the cathode materialis increased.

Example 3

FIG. 5 illustrates Example 3 of the present invention. In FIG. 5,numeral 20 denotes a substrate, and 21 a lower face electrode, 22 anorganic compound layer, and 23 an upper face electrode, respectively.

The substrate 20 was subjected to ultrasonic cleaning successively withacetone and isopropyl alcohol (IPA), and then cleaned in boiling IPA anddried. Then, using a vacuum evaporation apparatus (manufactured byShinku-Kiko, Inc.), on the cleaned substrate, aluminum (Al) wasevaporated by the vacuum evaporation method to form a film of 50 nm inthickness as the lower face electrode 21. The evaporation conditionswere that the degree of the vacuum when evaporating was 1.0×10⁻⁶ Torr,and the evaporation rate was from 1 nm/sec. Then, Alq3 and cesiumcarbonate (Cs₂CO₃) were evaporated to form a film of 50 nm in thicknessas the organic compound layer 22 in such a way that the evaporationrates of the Alq3 and cesium carbonate were respectively so adjustedthat the Alq3 and cesium carbonate were mixed in the film thicknessratio of 9:1. Finally, aluminum (Al), the same material as that for thelower face electrode, was evaporated to form the upper face electrode 23in a thickness of 150 nm on the organic compound layer 22 under thecondition of the evaporation rate of 1 nm/sec. The molar ratio of Alq3to cesium carbonate in the organic compound layer was 1:0.57.

In this way, there was obtained an element which was provided, on thesubstrate 20, with the lower face electrode 21, organic compound layer22, and upper face electrode 23. While a direct current voltage wasapplied to the element between either the lower face electrode 21 as theanode and the upper face electrode 23 as the cathode or between thelower face electrode 21 as the cathode and the upper face electrode 23as the anode, the voltage-current characteristics were measured.Consequently, at an applied voltage of 10 V, the current density wasrespectively 2250 mA/cm² in the case of using the upper face electrode23 as the cathode and 1960 mA/cm² in the case of using the lower faceelectrode 21 as the cathode.

Comparative Example 7

In the present comparative example, lithium fluoride was used in placeof a carbonate used in Example 3.

Under the conditions similar to those in Example 3, an aluminum (Al)film of 50 nm in thickness is formed as the lower face electrode 21, andthereon Alq3 and lithium fluoride (LiF) were evaporated to form a filmof 50 nm in thickness as the organic compound layer 22 in such a waythat the evaporation rates of the Alq3 and lithium fluoride wererespectively so adjusted that the Alq3 and lithium fluoride were mixedin the film thickness ratio of 9:1. Finally, aluminum (Al) wasevaporated to form the top face electrode 23 in a thickness of 150 nm onthe organic compound layer 22 under the condition of the evaporationrate of 1 nm/sec.

In this way, there was obtained an element which was provided, on thetransparent substrate 20, with the lower face electrode 21, organiccompound layer 22, and upper face electrode 23. While a direct currentvoltage was applied to the element between either the lower faceelectrode 21 as the anode and the upper face electrode 23 as the cathodeor between the lower face electrode 21 as the cathode and the upper faceelectrode 23 as the anode, the voltage-current characteristics weremeasured. Consequently, at an applied voltage of 10 V, the currentdensity was respectively 935 mA/cm² in the case of using the upper faceelectrode 23 as the cathode, and 11 mA/cm² in the case of using thelower face electrode 21 as the cathode.

From the results in Comparative Example 7, it can be seen that in theelement in which lithium fluoride (LiF) is used in the organic compoundlayer, the current flows when electrons are injected from the side ofthe upper face electrode 23 formed as a film subsequent to the formationof the organic compound layer, but electrons can be scarcely injectedfrom the side of the lower face electrode 21 formed beforehand. Alight-emitting element having an electrode formed in a heated state onthe organic compound layer containing a salt can make the current flowsatisfactorily.

On the other hand, in the element according to the present invention inwhich cesium carbonate (Cs₂CO₃) is used in the organic compound layer22, it is possible that the barrier for electron injection from thecathode is reduced and thereby the driving voltage of the element islowered without being subjected to any restrictions in the order of thefilm formation for the organic compound layer 22 and the electrode incontact with the organic compound layer.

Example 4

The present example is an example of an element in which an additionallayer (block layer) is provided between the organic compound layer andthe light-emitting layer.

The element of Example 4 is illustrated in FIG. 6. In FIG. 6, numeral 30denotes a transparent substrate on the anode side, and 31 the ITO layeras an anode electrode for use in hole injection, 32 a hole transportinglayer, 33 a light emitting layer, 34 a block layer, 35 an organiccompound layer, and 36 a cathode.

Under the conditions similar to those in Example 1, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 32 on the ITO as the anode electrode 31, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 33. Then, a10 nm thick film of bathophenanthroline (hereinafter, referred to as“B-phen”) represented by the following chemical formula 3:

was formed as the block layer 34. Successively, B-phen and cesiumcarbonate (Cs₂CO₃) were evaporated to form a film of 25 nm in thicknesson the block layer 34 as the organic compound layer 35 in such a waythat the evaporation rates of the B-phen and cesium carbonate wererespectively so adjusted that the B-phen and cesium carbonate were mixedin the film thickness ratio of 8.8:1.2. Finally, a film of aluminum (Al)of 1.50 nm in thickness was evaporated as the cathode electrode 36 onthe organic compound layer 35 under the condition of the evaporationrate of 1 nm/sec. The molar ratio of the B-phen to Cs₂CO₃ in the organiccompound layer 35 was 1:0.5.

In this way, a light-emitting element was obtained by providing, on thetransparent substrate 30, the anode electrode 31, hole transportinglayer 32, light-emitting layer 33, block layer 34, organic compoundlayer 35, and cathode electrode 36. Successively, while a direct currentvoltage was applied between the ITO as the anode electrode 31 and thealuminum as the cathode electrode 36 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 44105 cd/m²and a current density of 1760 mA/cm² at an applied voltage of 9 V. Inaddition, the element exhibited a maximum efficiency of 2.3 lm/W at anapplied voltage of 5 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 7.

Comparative Example 8

In the present comparative example, the block layer 34 used in Example 4is not used.

Under the conditions similar to those in Example 4, at the beginning, anα-NPD film of 35 nm in thickness was formed as the hole transportinglayer 32 on the ITO as the anode electrode 31, and thereon an Alq3 filmof 15 nm in thickness was formed as the light-emitting layer 33.Successively, B-phen and cesium carbonate (Cs₂CO₃) were evaporated toform a film of 35 nm in thickness on the light-emitting layer 33 as theorganic compound layer 35 in such a way that the evaporation rates ofthe B-phen and cesium carbonate were respectively so adjusted that theB-phen and cesium carbonate were mixed in the film thickness ratio of8.8:1.2. Finally, a film of aluminum (Al) of 150 nm in thickness wasevaporated as the cathode electrode 36 on the organic compound layer 35.

In this way, a light-emitting element was obtained by providing, on atransparent substrate 30, the anode electrode 31, hole transportinglayer 32, light-emitting layer 33, organic compound layer 35, andcathode electrode 36. Successively, while a direct current voltage wasapplied between the ITO as the anode electrode 31 and the aluminum asthe cathode electrode 36 in the light-emitting element, thelight-emitting characteristics of the element were measured.Consequently, the element exhibited a maximum luminance of 33751 cd/m²and a current density of 3223 mA/cm² at an applied voltage of 11 V. Inaddition, the element exhibited a maximum efficiency of 1.01 lm/W at anapplied voltage of 5 V. The voltage-luminance characteristic of thelight-emitting element is shown in FIG. 7.

The light-emitting element shown in Example 4 which has the block layerand the organic compound layer containing cesium carbonate is improvedlargely in the light-emitting efficiency as compared to the element inComparative Example 8 in which no block layer was provided. The blocklayer has a function blocking the transit of the holes injected from theanode electrode through the light-emitting layer, and is a layer forpreventing the penetration of the carbonate, contained in the organiccompound layer superposed on the block layer, into the light-emittinglayer. If the carbonate penetrates the light-emitting layer, thepenetrated portions do not emit light so that the light-emittingefficiency is degraded. Incidentally, the reason why the carbonatecontained in the organic compound layer penetrates the light-emittinglayer is not clear at present; the present inventors conceive that thecarbonate gets into the light-emitting layer owing to the energyacquired through film formation.

It can be seen that such a block layer is effective in improving thelight-emitting efficiency which block layer is provided between thelight-emitting layer and the organic compound layer, blocks the transmitof the holes through the emitting layer, and prevents the penetration ofthe carbonate used in the organic compound layer into the light-emittinglayer.

As described above with reference to

Embodiments and Examples, according to the present invention, there canbe provided a light-emitting element in which the organic compound layercomposed of a carbonate easy in handling and an organic compound is incontact with the cathode.

1. A light-emitting element comprising: a pair of electrodes consistingof an anode and a cathode; an organic compound layer provided betweensaid pair of electrodes; and a light-emitting layer provided betweensaid organic compound layer and said anode, wherein said organiccompound layer in substantially electrical contact with said cathodeelectrode is composed of at least an organic compound and a carbonateand is a co-evaporation layer.
 2. The light-emitting element accordingto claim 1, wherein a molar ratio of said organic compound to saidcarbonate in said organic compound layer is in a range from 1:0.01 to1:100.
 3. The light-emitting element according to claim 2, wherein saidmolar ratio is in a range from 1:0.4 to 1:0.6.
 4. The light-emittingelement according to claim 1, wherein a cation of said carbonate is analkali metal ion or an alkaline earth metal ion.
 5. The light-emittingelement according to claim 1, wherein said carbonate is cesiumcarbonate.
 6. The light-emitting element according to claim 1, whereinsaid carbonate is lithium carbonate.
 7. The light-emitting elementaccording to claim 1, wherein said cathode is transparent to visiblelight.
 8. The light-emitting element according to claim 1, wherein saidcathode is an ITO electrode.
 9. The light-emitting element according toclaim 1, wherein said cathode is an electrode composed of any one ofgold, silver, and aluminum.
 10. The light-emitting element according toclaim 1, wherein another organic compound layer is provided between saidorganic compound layer and said light-emitting layer.
 11. Thelight-emitting element according to claim 10, wherein another organiccompound layer is provided between said organic compound layer and saidlight-emitting layer.
 12. The light-emitting element according to claim1, wherein light is brought out from said cathode.
 13. A light-emittingappratus comprising on its surface a plurality of the light-emittingelements according to claim
 1. 14. The light-emitting apparatusaccording to claim 13, wherein the light-emitting apparatus is aninformation displaying part of a display.
 15. The light-emittingapparatus according to claim 13, wherein said light-emitting appratus isan apparatus for exposing a photoreceptor in an electrophotographicimage formation apparatus.
 16. A method of producing a light-emittingelement comprising (i) a pair of electrodes consisting of an anode and acathode, (ii) an organic compound layer provided between said pair ofelectrodes and (iii) a light-emitting layer provided between the organiccompound layer and the anode, said method comprising providing alight-emitting layer on a side of the anode, an organic compound layerforming step of providing, on a side of the cathode, at least an organiccompound and a carbonate for constituting said organic compound layer toform the organic compound layer, and a step of providing a cathode tocome into electrical contact with said organic compound layer, whereinsaid organic compound layer forming step is a step of forming theorganic compound layer by co-evaporation of the organic compound and thecarbonate.
 17. The method according to claim 16, wherein a molar ratioof said organic compound to said carbonate is in a range from 1:0.01 to1:100.
 18. The method according to claim 17, wherein said molar ratio isin a range from 1:0.4 to 1:0.6.
 19. The method according to claim 16,wherein in said organic compound layer forming step, the carbonate andorganic compound are co-evaporated at a temperature not higher than 700°C.
 20. The method according to claim 16, further comprising a secondorganic compound layer forming step to form a second organic compoundlayer between said organic compound layer and said light-emitting layer.21. The method according to claim 20, wherein said second organiccompound layer is at least any one of an electron transporting layer anda hole blocking layer.
 22. A light-emitting element comprising: (a) apair of electrodes consisting of an anode and a cathode, (b) alight-emitting layer, and (c) an organic compound layer provided betweensaid pair of electrodes, wherein said organic compound layer inelectrical contact with said cathode electrode is composed of at leastan organic compound and a carbonate and said cathode is transparent tovisible light.
 23. The light-emitting element according to claim 22,wherein a second organic compound layer is provided between said organiccompound layer and said light-emitting layer.
 24. The light-emittingelement according to claim 23, wherein said second organic compoundlayer is at least any one of an electron transporting layer and a holeblocking layer.
 25. A method of producing a light-emitting elementcomprising (i) a pair of electrodes consisting of an anode and acathode, (ii) a light-emitting layer and (iii) an organic compound layerprovided between the pair of electrodes, which method comprises: (a)forming the organic compound layer on a side of the cathode; and (b)causing the cathode to come into electrical contact with the organiccompound layer wherein the organic compound layer is formed byevaporation of an organic compound and a carbonate.
 26. The methodaccording to claim 25, wherein in the organic compound layer formingstep, the carbonate is evaporated at a temperature not higher than 700°C.
 27. The method according to claim 25, further comprising a secondorganic compound layer forming step to provide a second orgnaniccompound layer between said organic compound layer and saidlight-emitting layer.
 28. The method according to claim 27, wherein saidsecond organic compound layer is at least any one of an electrontransporting layer and a hole blocking layer.